Susceptibility to asthma exacerbations – antiviral immunity and protection against asthma exacerbations

Epidemiological studies show that respiratory virus infections are associated with the great majority of asthma exacerbations in both adults and in children (1-6) and that rhinoviruses are the most frequent viruses detected (1). Rhinoviruses infect the lower airways (7) and, in experimental studies, rhinovirus infection leads to long lasting airway narrowing (8), reductions in lung function in asthmatic volunteers (9) and to increased bronchial hyperreactivity in allergic subjects (10). A recent study indicates that asthmatic subjects are more susceptible to rhinovirus infection than normal subjects in that they develop more severe and more prolonged lower respiratory tract symptoms and falls in lung function following naturally occurring rhinovirus infection than do normal individuals (2). The reasons for this increased susceptibility in asthmatics are still largely unknown. An emerging hypothesis to explain the increased clinical susceptibility of asthmatic patients to viral infection is that asthmatic subjects have an impaired antiviral immunity in terms of innate immune and acquired immune responses. Therefore we will review antiviral mechanisms in normal subjects, emphasizing data on antiviral defence against respiratory viruses [mainly rhinovirus and respiratory syncytial virus (RSV)] and then we will review the relatively few available data specifically investigating immune responses of asthmatic patients to respiratory viral infections. Identification of differences between normal and asthmatic subjects in terms of antiviral responses to respiratory virus infections may identify novel pharmacological approaches for prevention and/or to treatment of virus-induced asthma exacerbations